1. Field of the Invention
The present invention relates to an exhaust passage control valve disposed in an exhaust passage of an internal combustion engine (e.g., an engine of a vehicle). Specifically, the present invention relates to an exhaust passage control valve that opens when pressure of exhaust gas flowing through the exhaust passage is equal to or exceeds a predetermined level.
2. Description of the Related Art
An exhaust passage control valve is disposed in an exhaust passage of an internal combustion engine. The exhaust passage control valve opens when pressure of exhaust gas flowing through the exhaust passage is equal to or exceeds a predetermined level. For example, a muffler is disposed in an exhaust device of a vehicle engine. A bypass passage is formed within the muffler for reducing the air-flow resistance, and the exhaust passage control valve is disposed within the bypass passage. When the pressure of the exhaust gas is high, the exhaust passage control valve opens, and engine output is thus increased. When the pressure of the exhaust gas is low, the exhaust passage control valve closes, and muffler performance thus improves.
Conventionally, a butterfly valve is used within this type of exhaust passage control valve. In butterfly valves, spring load increases as the degree of opening of the valve increases. As a result, even if the butterfly valve starts to open when the pressure of the exhaust gas reaches the predetermined level, the pressure of the exhaust gas must become considerably higher than the predetermined level for the butterfly valve to open fully. In order to deal with this problem, an exhaust passage control valve disclosed in Japanese Patent No. 3326746 has been proposed.
This exhaust passage control valve comprises a housing through which exhaust gas from the engine flows, a valve member mounted on the housing, and a helical torsion spring biasing the valve member towards the closing position. The helical torsion spring is disposed at the opposite side of the valve member from the housing side thereof. A coil part of the helical torsion spring is supported by a supporting part formed at approximately the center of the valve member. A center axis of the coil part is approximately parallel with a surface of the valve member. Arms of the helical torsion spring are supported in a spring mounting member. The arms of the helical torsion spring can be slid in a longitudinal direction with respect to the spring mounting member. The spring mounting member is fixed to the housing. When these components (i.e., housing, valve member, torsion coil member, and spring mounting member) have been assembled, the arms of the helical torsion spring change position by rotating with respect to the coil part. When the arms rotate, the arms bend with respect to the coil part. The valve member is biased towards the closing side by the bending counter-force of the arms.
With this exhaust passage control valve, when the valve member moves to an opening side, the arms of the helical torsion spring slide in the longitudinal direction with respect to the spring mounting member. When the arms slide with respect to the spring mounting member, there is a change in the distance from the center of the coil part to an arm mounting position (i.e., a change in the effective length of the arms). When the effective length of the arms increases, the bending counter-force of the arms becomes smaller. The exhaust passage control valve is set such that the effective length of the arms increases as the arms move towards the opening side. It is therefore possible to prevent an increase in the spring load with respect to the degree of opening of the valve. As a result, the valve fully opens rapidly when the pressure of the exhaust gas exceeds the predetermined level, and the valve is able to open sufficiently.